Cryptocurrencies are digital assets that use cryptography to enable secure transactions and the control of mining (creation of more digital currency).
They were invented by a group of people that believed transactions should not need a central server or authority, but rather, be shared, peer-to-peer.
This is else known as restricted entries in a database and subsequently the solution to the “double-spend” problem, i.e. sending the same Cryptocurrency to two people.
Bitcoin is a Cryptocurrency using the above technology.